On Fri, Mar 19, 2010 at 8:28 PM, Dan Naumov <dan.naumov at gmail.com> wrote:
> Oh my god... Why did noone tell me how much of an enormous performance
> boost vfs.zfs.prefetch_disable=0 (aka actually enabling prefetch) is.
> My local reads off the mirror pool jumped from 75mb/s to 96mb/s (ie.
> they are now nearly 25% faster than reading off an individual disk)
> and reads off a Samba share skyrocketed from 50mb/s to 90mb/s.
>> By default, FreeBSD sets vfs.zfs.prefetch_disable to 1 on any i386
> systems and on any amd64 systems with less than 4GB of avaiable
> memory. My system is amd64 with 4gb ram, but integrated video eats
> some of that, so the autotuning disabled the prefetch. I had read up
> on it and a fair amount of people seemed to have performance issues
> caused by having prefetch enabled and get better results with it
> turned off, in my case however, it seems that enabling it gave a
> really solid boost to performance.
>
My home VBox server is similar specs and I enabled the prefetch from the
start. A few days ago, I added an intel SSD as the zpool cache device and
the read speed is mind blowing now. This is from inside a VM frunning on it
meaning ad0 is really a vdi. Once the cache is populated, HD latency is
mostly a thing of the past.
# diskinfo -tv /dev/ad0
/dev/ad0
512 # sectorsize
12884901888 # mediasize in bytes (12G)
25165824 # mediasize in sectors
24966 # Cylinders according to firmware.
16 # Heads according to firmware.
63 # Sectors according to firmware.
VBf9752473-05343e4e # Disk ident.
Seek times:
Full stroke: 250 iter in 0.082321 sec = 0.329 msec
Half stroke: 250 iter in 0.078944 sec = 0.316 msec
Quarter stroke: 500 iter in 0.161266 sec = 0.323 msec
Short forward: 400 iter in 0.128624 sec = 0.322 msec
Short backward: 400 iter in 0.131770 sec = 0.329 msec
Seq outer: 2048 iter in 0.667510 sec = 0.326 msec
Seq inner: 2048 iter in 0.691691 sec = 0.338 msec
Transfer rates:
outside: 102400 kbytes in 0.722864 sec = 141659 kbytes/sec
middle: 102400 kbytes in 0.813619 sec = 125857 kbytes/sec
inside: 102400 kbytes in 0.838129 sec = 122177 kbytes/sec
--
Adam Vande More